Germfree rats and mice are being produced, maintained and used for (a) the development of germfree technology, (b) the development of model disease systems with unique attributes, and (c) provision of germfree technology and animals to biomedical investigators. Thus far, model systems have been developed for leukemias, reticulum cell sarcoma, and for carcinomas of intestine, prostate, breast, and lung. These are being used for the development of prophylactic and therapeutic procedures. Intramural and extramural investigators have been provided advice, assistance and animals for germfree research.